1. Field of the Invention
The present invention, in general, relates to telescopes and, more particularly, to a variable field of view telescope that is adapted for simultaneous use with visible and infrared wavelengths.
Cassegrain types of telescopes are well known reflector-types of telescopes that are used in both the recreational arts for astronomy and ground observation and also for commercial purposes as well as for certain military applications. In general, they offer a longer focal length in a more compact package. It is desirable with all optical systems to obtain as much light energy as possible (i.e., to provide a large aperture) in as compact and rigid a structure as is possible.
Variable field of view telescopes that include a plurality of field of view optical groups for insertion into the optical path are also known. The known prior art devices utilize a “C” structure that extends in an arc from the base of a primary mirror to a position over the primary mirror. A turret attached to the upper end of the C structure is used to suspend the plurality of optical groups a predetermined distance over the primary mirror.
The plurality of optical groups that are disposed on the turret are arranged for sequential insertion of one group into the field of view following the simultaneous withdrawal of a preceding group from the field of view. Accordingly, one optical group is always disposed in the field of view.
While providing the benefit of being able to withdraw and then insert one of several optical groups into the field of view (to affect the field of view), there are several disadvantages inherent with this type of design. An important disadvantage is that the C structure incurs considerable flexing that is an inherent characteristic due to the length of the arm and the fact that it is supported only at the base of the structure.
This can cause misalignment of the optical groups and resultant distortion in the optical path. Also, because of the long length of the C structure, thermal expansion and contraction effects can further contribute to misalignment and error along the optical path or limit the temperature range in which the equipment can properly function.
Also, the C structure obstructs a significant quantity of light energy that would otherwise impede upon the primary mirror. The C structure provides only one point of mechanical support. Therefore, it must be mechanically large and strong enough so as to sufficiently limit flexing and vibration. Because the structure is supported on the outside of the primary mirror, a long moment arm is also created from the base of the C structure to the turret. It is the long moment arm that is subject to flexing and the turret that is subject to vibration.
Vibration can be in response to any mechanical energy that the C structure experiences. For example, vibration of the engine(s) that propel the vehicle or aircraft may be transmitted to the C structure. While there is always the possibility for vibration of the moment arm, there is also the possibility that the length of the arm can resonate in a harmonic frequency thereby increasing the amplitude of vibration.
To minimize flexing and vibration of the C structure design, it is built as large and as strong as it needs to be in order to function in any given environment. However, this is not desirable because it is preferable that the C structure be as small as possible so as to minimize the amount of light energy that it obstructs. Accordingly, a tradeoff is made in the prior art design that sacrifices light gathering ability for necessary rigidity.
Also, the larger the C structure is, the heavier it also becomes. This is another limitation that is an especially important consideration in various circumstances, for example, when the telescope is used in an aircraft or spacecraft. While it is desirable that a telescope be lightweight, this is crucial in certain applications.
The prior art design also relies upon the use of moveable hard stops to repeat the position that the plurality of optical groups are maintained in. This limits the speed by which optical groups can be changed without causing damage.
Furthermore, when any of the plurality of optical groups are moved into and out of the optical path, it is desirable that precise repeatability of their position occur, else the optical path is adversely affected. The prior art designs that require variability in the stops are unable to provide optimum repeatability in the positioning of the optical groups.
An especially significant limitation with the prior art designs is that the field of view changes in steps according to the number and optical characteristics of the various optical groups. It is desirable to be able to step in IR and also to provide a progressive zoom capability in visible as well, which the prior art fails to provide.
The prior art designs are also limited in their ability to provide both IR (infrared) and visible light gathering ability. Separate optical paths are typically required. If the telescope is used in a gimbal ball, for example, this would result in having two apertures in the gimbal ball, one for visible and one for IR. This is undesirable for many reasons.
There are many light frequencies that may be of interest. It is desirable to be able to view two or more channels simultaneously and also to change the field of view (FOV) from between wide angle FOV (lower magnification) and narrow FOV (higher magnification) capabilities. Prior art designs have been unable to effectively switch back and forth between infrared and visible light while also providing a zoom capability in visible.
For many applications it is desirable to be able to zoom in the visible spectrum. For example, if after detecting an object of interest using a wide field of view (in either the visible spectrum or IR spectrum), it is desirable to then be able to narrow the field of view and zoom in to complete a closer examination of the object. If the object of interest was first discovered as a presence in the infrared spectrum, it may be desirable to switch to the visible spectrum and zoom in accordingly.
It is also desirable to be able to gather data in both spectrums simultaneously, for example in a narrow FOV in IR and in visible. Ideally, the data in both wavelengths could be recorded for future comparison and subsequent analysis. It is desirable then that a recording camera or some other type of transducer (i.e., a focal plane array; FPA) be placed in the field of view, as desired.
Prior art designs have provided limited zoom capability in the visible spectrum. This is because a rigid and relatively long physical path is required to house the optical elements that typically accompany an optical zoom system.
Furthermore, space is often a commodity in short supply, especially if the telescope is to be housed in a gimbal ball.
Accordingly, it is desirable to be able to provide a space to accommodate the optical elements of a visible zoom optical system and do so in such manner as to provide an optimally long linear path that is especially rigid. It would also be a significant and unexpected benefit if the structures used for the visible optical zoom system were able to provide increased rigidity to the overall structure for supporting an optical telescope.
As mentioned briefly hereinabove, there are many possible frequencies of light (electromagnetic) energy that may of interest depending upon the application. It is desirable to provide versatility in such an instrument to change the transducers and analyze the relevant spectrums that may be of interest.
The structural limitations that have, heretobefore, prevented the design of such an optical system have been satisfied by the disclosure of the related patent application, herein incorporated by reference. Even so, an optical system that provides such versatility has been previously unavailable and it is the subject of the instant disclosure to describe such capability.
Accordingly, there exists today a need for an optical telescope that helps ameliorate the above-mentioned problems and limitations.
Clearly, such an apparatus would be an especially useful and desirable device.
2. Description of Prior Art
Telescopes and zoom lenses are, in general, known. For example, the following patent describes various types of these devices:
U.S. Pat. No. 5,907,433 to Voigt et al, May 25, 1999; and
U.S. Pat. No. 5,940,222 to Sinclair et al, Aug. 17, 1999.
While the structural arrangements of the above described devices, at first appearance, have similarities with the present invention, they differ in material respects. These differences, which will be described in more detail hereinafter, are essential for the effective use of the invention and which admit of the advantages that are not available with the prior devices.